Conventional parallel-operating power supply systems have a problem of how to suppress cross current flowing between power supply apparatuses. According to a technology disclosed in JP2010-158125A1, for example, cross current control is achieved in the following manner. A common-mode choke coil is disposed between first and second switching power supplies and a load in such a manner that the cross current flowing from the first switching power supply to the second switching power supply and the current flowing from the load to the first switching power supply can flow, as a common current, into the common-mode choke coil. Further, a second common-mode choke coil is disposed between the first and second switching power supplies and the load in such a manner that the cross current flowing from the second switching power supply to the first switching power supply and the current flowing from the load to the second switching power supply can flow, as a common current, into the second common-mode choke coil.
Recent switching power supplies are frequently digitally controlled. A control unit ON-OFF controlling a switching device of a switching power supply provides a switching signal for achieving the ON-OFF control based on a reference clock signal. When such switching power supplies are operated in parallel, and if each switching power supply is arranged to provide its own reference clock signal for itself, the respective reference clock signals may become out of phase with each, causing the timing of turning on and off the switching devices of the respective switching power supplies to be deviated from each other, which, in turn, may result in cross current. In order to suppress ripple components in the current to be supplied to the load, the timing of switching the switching device of each of the switching power supplies must be controlled with precise difference provided with respect to each other, but such control is difficult if the phases of the reference clock signals of the respective switching power supplies are different.
An object of the invention is to provide a parallel-operating power supply system in which occurrence of cross currents can be suppressed and also ripple components in the output thereof can be reduced.